Update MSRV to 1.81

[waywardmonkeys]

This also changes to use the derived Default for enums since that is available with a later MSRV (and would otherwise have clippy warnings).

Updating the MSRV allows the no_std support to be slightly better by allowing core::error::Error to be impl'd for the error types.

[waywardmonkeys]

@madsmtm Even if we don't do this bump now ... I could update the MSRV here to 1.73 (matching winit) which would let us keep these Default changes, but also letting CI run slightly faster by not needing to do a build without the sparse registries in Cargo (stabilized in Rust 1.68, IIRC).

[pyfisch]

@madsmtm Either we bump the MSRV to 1.81 before the v0.8 release or it stays on 1.61 for a long time as this crate doesn't have too many releases.

[waywardmonkeys]

I've updated this now that no_std has landed so that the workaround for core::error::Error is addressed. If this lands, then I'll have one more follow up on the no_std stuff to make it a bit better.

[madsmtm]

I'd prefer to not update MSRV.

Like I said in #17, I'd rather have two MSRVs, one for no_std, and one for std (see #75). Or just one MSRV, and then not have the Error impl in no_std (as is currently implemented).